The use of metal salts, especially zinc salts, of phosphorodithioic acids as antioxidants and extreme pressure agents in lubricants and functional fluids has been known for some time. However, the environment in which said lubricants and functional fluids are used has become increasingly severe over recent years with the further development of machinery employing such lubricants and functional fluids. It is important, therefore, that materials of this type be developed which have higher thermal and hydrolytic stability than has previously been the case.
The use of sulfurized calcium alkyl phenates as compounding agents in lubricating oils to inhibit corrosion, piston ring sticking and gum formation in internal combustion engines resulting from oxidation of lubricating oil and polymerization of engine fuel residues is also known, as indicated in U.S. Pat. Nos. 2,680,096 and 3,036,971.
Metal salts of mixtures of phosphorodithioic acids and carboxylic acids have been described as being useful in lubricants and functional fluids in U.S. Pat. No. 4,308,154. In copending U.S. application Ser. No. 334,251, filed on Dec. 24, 1981, phosphorus- and sulfur-containing compositions of improved thermalstability useful in lubricants and functional fluids are described. The compositions comprise a metal salt of a mixture of a phosphorodithioic acid and an aliphatic or alicyclic carboxylic acid, and at least one sulfurized Group II metal phenate.
An improvement in the properties of salts of dialkylphosphorodithioic acids is described in U.S. Pat. No. 4,263,150 wherein the dialkylphosphorodithioic acids or their salts are treated with phosphites, especially triaryl phosphites. The products obtained by this process exhibit reduced tendency to stain and corrode metal parts, especially copper parts, when incorporated into lubricants and functional fluids. The process also is useful for treating metal salts of mixtures of dialkylphosphorodithioic acid and carboxylic acids.